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1. Technical Field
The present invention relates generally to communication devices for coupling to an external transmission medium; and, more particularly, it relates to an efficient method for regulating the transfer of power across a high voltage isolation barrier.
2. Related Art
Communication devices coupled to a telephone line or similar transmission medium generally must adhere to rigid electrical isolation requirements. Subscriber equipment or data communications equipment (DCE), such as data modems, typically provide for some form of electrical isolation to prevent voltage surges or transients originating from the subscriber equipment or lightning from having a deleterious effect on the telephone network and vice versa. Electrical isolation also addresses potential problems sometimes associated with differences in operating voltages between a telephone line and the subscriber equipment. More particularly, telephone line voltages may vary widely across a given network, and often exceed the operating voltage of subscriber equipment. In the United States, 1,500 volt isolation is currently required. In other countries, the prescribed isolation may reach 3,000-4,000 volts.
Isolation transformers are often employed to magnetically couple signals between a two-wire telephone line and the analog front end of a modem or other circuit while maintaining an appropriate level of electrical isolation. The isolation transformer functions to block potentially harmful DC components, thereby protecting both sides of the data connection.
The isolation transformer is typically part of what is referred to in the modem arts as a data access arrangement (DAA). The term DAA generally indicates circuitry which provides an interface between a public telephone network originating in a central office (CO) and a digital data bus of a host system or data terminal equipment (DTE). In addition to electrical isolation, the DAA may develop a number of signals (e.g., a ring signal) for provision to subscriber equipment. The DAA generally receives signals from the phone line through a telephone jack, such as a RJ11C connection as used for standard telephones.
Some modem configurations utilize a DAA having line side circuitry including telephone network interface and system side circuitry including a host system interface, the line side circuitry and system side circuitry being separated by the high voltage isolation barrier. Power for line side circuitry may be communicated from the host system across an isolation transformer or other isolation barrier. Difficulties arise in such a system, however, in minimizing power dissipation due to losses typically incurred in transferring power across the high voltage isolation barrier of a DAA. The inherent inefficiencies in prior transformer and regulator circuitry requires that the system side supply more power than actually required by the line side circuitry to detect and validate ring signals or to perform other communications functions. Such inefficiencies may be particularly problematic in a xe2x80x9cwake-on-ringxe2x80x9d or similar mode of operation in which power consumption specifications are very stringent, or when the current consumption of the line side circuitry is variable. Additionally, variations in voltage levels on the system side further complicate the transfer of power across the high voltage isolation barrier. The power transfer overhead needed to account for such variations may compound the effect of inefficiencies in the power transfer mechanism. Therefore, an improved method of transferring power across a high voltage isolation barrier is needed.
Briefly, the present invention provides an efficient method and circuitry for transferring various levels of power across a high voltage isolation barrier such as an isolation transformer. In one embodiment of the invention, a DAA is provided with system side circuitry that provides power to line side circuitry via the high voltage isolation barrier. The line side circuitry includes voltage measurement circuitry, such as an analog-to-digital converter, that periodically measures the line side supply voltage. The resultant information is transmitted across the high voltage isolation barrier to the system side circuitry, where control circuitry compares the digital representation of the measured voltage level to a predetermined value. Based on the results of this comparison, the control circuitry may alter the amount of power transferred across the high voltage isolation barrier to the line side circuitry in order to optimize power transfer efficiency.
In one embodiment of the invention, a regulator is provided in series between an isolation transformer and the line side circuitry for purposes of regulating the supply voltage provided to the line side circuitry. The analog-to-digital converter of the line side circuitry may be configured to measure either the regulated or unregulated supply voltage, with corresponding modifications to a power management process. Further, the amount of power transferred across the isolation transformer may be varied in accordance with the invention by varying the frequency and/or duty cycle of differential transformer driver signals. In an alternate embodiment of the invention, a single-ended pulse is applied to the primary side of the transformer to effectuate power transfer.
The analog-to-digital converter can be shared with other resources of the line side circuitry, such that operation of the analog-to-digital converter can be time-multiplexed for various functions, thereby minimizing the cost of the line side circuitry. In addition, the power management process may include a certain degree of hysterisis to prevent instability in the closed loop system.
Thus, the method and circuitry of the present invention reduce unnecessary power consumption that may result from variations in the supply voltage of the system side and variations in the current consumption of the line side circuitry of a DAA. Further, inefficiencies due to variations in the isolation barrier and regulator circuitry are mitigated.